Plain bearing materials require good surface properties since they must slide against the mating surface without causing wear to either surface and without `seizing` i.e. welding to the mating surface. This property usually requires that the alloy is soft and has a low melting point, or contains a low melting point constituent. The alloy also needs to be capable of carrying the load imposed by the mating surface, which is often cyclic in nature, without break-up or fatigue of the bearing alloy. This property usually requires that the alloy be strong and strong alloys are usually hard. A compromise of property requirements in balancing both soft and hard attributes is necessary.
There are three well established aluminum alloys in use today as engine bearing materials each consisting of an aluminum material and a major alloying addition of a soft, low melting point metal. These three alloy systems are aluminum-cadmium, aluminum-tin and aluminum-lead.
The alloys of this invention are the result of a guest to develop an aluminum alloy with a much better combination of bearing properties than those alloys currently available.
According to the present invention, the bearing material comprises aluminum and 4% or more by weight of bismuth. The bismuth content by weight may be up to 8%, and for some purposes as much as 12%. The aluminum may or may not constitute the balance of the material, but will usually constitute at least 50% by weight of the material.
Aluminum-bismuth is an alternative system which has been shown by testing to have superior antiseizure properties to those alloys currently used. The higher the additions of bismuth the better the surface properties become, but there are practical limitations to the amount of bismuth which can be accommodated in an aluminum alloy by a casting process. Because of the liquid immiscibility of aluminum and bismuth, it is not feasible at acceptable temperatures to take more than about 8% of bismuth into solution. Higher bismuth alloys are best made by a sintering process.
Testing has shown, however, that even at bismuth contents as low as 4%-5%, the bearing surface properties are superior to the standard aluminum alloys currently in use. This surface property can be further enhanced by small additions of lead.
In order to impart sufficient wear resistance to the alloy, it is advisable to make small additions of an element or elements which produce a fine dispersion of hard particles in the aluminum matrix. An excellent element for this purpose is silicon which also improves the strength and surface properties.
Another important property of an engine bearing material is that it should withstand the cyclic loads imparted during operation. For this purpose a strengthening addition can be made to the aluminum alloy. One such strengthening addition is copper, which at small additions of around 1% by weight, imparts the necessary strength without deleteriously affecting the surface properties. Other strengthening and/or hard particle forming additives such as nickel, manganese, chromium, zinc, and/or antimony, could be made and small additions of tin could be included to improve corrosion resistance.
The resultant aluminum-bismuth family of alloys have the ability to run in engine bearing environments without the necessity of an expensive overlay plate of lead-tin alloy currently utilized on most engine bearings.